1. Field of the Invention
The present invention generally relates to capacitors and, more particularly, to high current capacitors.
2. Background of the Invention
Capacitors are widely used in electrical apparatus for different reasons. For example, capacitors can be used to store electrical charge and to generate a large electrical current and voltage, and the like.
Capacitors can be manufactured using different methods. More recently, capacitors, such as a metallized film capacitors, can be manufactured by wrapping two tightly wound sheets or sections around a core. Each sheet is composed of a dielectric film having a metallized layer disposed on one face of the film. The metallized layer extends to one edge of the face to provide an unmetallized edge. The unmetallized edges of the two sheets are placed opposite to each other when the sheets are stacked and wound together, such that only one metallized edge is available for connecting to a leas at each end of the rolled capacitor. Each end is sprayed with a conductive metal that bonds with the sheet having a metallized edge at that end. Leads are then attached to each sprayed end to form the capacitor electrode. The rolled capacitor is then placed in a housing and impregnated with a dielectric fluid. One example of the metallized film capacitors is disclosed in U.S. Pat. No. 4,897,761.
The metallized film capacitor can be used in an apparatus that provides a large electrical current or that is operated under a large electrical power. In a high current applications, a fault that occurs within the capacitor can cause a disaster. For example, the apparatus can catch a fire due to the fault. In addition, the fault in the capacitor can damage other electrical components of the apparatus. Accordingly, the ability to detect faults that occurs within the capacitor has become an important issue in the design of high current capacitors.
FIG. 1 shows a general structure of a typical metallized film capacitor 10. As shown, capacitor 10 includes metal housing 11, capacitor roll 12 mounted within metal housing 11, and dielectric fluid 15 that is present between metal housing 11 and capacitor roll 12. Inside of metal housing 11, there are two leads 13, that connects the capacitor roll 12 to two external terminals 14. Terminals 14 are used to connect to other electrical components of an electrical apparatus to form an electrical circuit (“apparatus circuit”). Metal housing 11 can be in a cylindrical, an oval, a circular or other desired shape. As known in the art, capacitor 10 can be used singly in the electrical apparatus. Several capacitors 10 can also be used together as an array of capacitors to provide a larger electrical current.
In high current applications, safety measures must be provided to detect a fault that could occur within the capacitor. The fault may include, for example, a excessive pressure or overheat condition within the capacitor housing, which is caused by an overload of the capacitor. The overload may cause a fire to an apparatus circuit which includes the capacitor or the array of capacitors. Conventionally, an internal fault interrupter is installed within metal housing 11. The fault interrupter breaks when a fault occurs within capacitor 10, thereby disconnecting the capacitor 10 from the apparatus circuit.
FIGS. 2A and 2B show a conventional capacitor design that utilizes an internal fault interrupter. Such capacitor design is commonly used in the United States for AC capacitors.
With reference to FIGS. 2A and 2B, the internal fault interrupter is installed within metal housing 11 on the top of capacitor roll 12. Capacitor 10 includes insulating barrier 21 on which leads 13 are extended from capacitor roll 12 are welded. Capacitor 10 further includes a contact plate 22 that is set on the top of insulating barrier 21. The contact plate 22 is contacted with leads 13 on one side and with external terminals 14 on the other side. In this design, external terminals 14 are mounted on contact plate 22 by means of a rivet or a screw 23. When capacitor 10 is operated normally, contact plate 22 rests on insulation barrier 21 so that the contact plate 22 is in contact with both leads 13 and external terminals 14 to provide a electrical circuit. When a fault occurs within capacitor 10, the excessive pressure resulted from the fault forces contact plate 22 to expanded outwardly (or upwardly as depicted in FIG. 2B) to move away from leads 13. When contact plate 22 is expanded, it is no longer in contact with leads 13, resulting in a disconnection of the electrical circuit.
The capacitor shown in FIGS. 2A and 2B, however, is not suitable for providing greater than 15 amps RMS continuous duty. This is because a weak weld between insulation barrier 21 and leads 13 and the utilization of rivets 23 to mount terminals 14 on contact plate 22 are not capable of processing current greater than 15 amps.
FIGS. 3A and 3B illustrate another capacitor having an internal fault interrupter. Capacitor 10 as shown in FIG. 3A includes a bellows 31 that is fabricated on a top side of housing 11 and at least one “notched” wire conductor 33 within housing 11 which replaces conventional leads 13 for connecting external terminals 14 with capacitor roll 12. When capacitor 10 operates normally, bellows 31 maintains a non-expanded position as shown in FIG. 3A and notched wired conductor 33 properly connects capacitor roll 12 with external terminals 14. When an excessive pressure occurs resulted from a fault within capacitor 10, bellows 31 expands in an axial direction of housing 11. The expansion forces notched wire conductor 33 to be broken, as shown in FIG. 3B.
Although capacitor 10 shown in FIGS. 3A and 3B provides more current duty than that shown in FIGS. 2A and 2B, the current is still limited by notched wire conductor 33.
Capacitor 10 shown in FIG. 4 includes a high current interrupter. In FIG. 4, capacitor 10 includes bellows 41 fabricated on a top side of housing 11 and plug-type conductors 42 within housing 11. As shown, the top ends of plug-type conductors 42 connect to external terminals 14. Connector 43 having a number of sockets 431 is attached to capacitor roll 12 which, when capacitor 10 operates normally (see dashed lines), receives the bottom ends of plug-type conductors 42 within sockets 431. In this manner, capacitor roll 12 is electrically connected with an apparatus circuit by external terminal 14. On the contrary, as shown by solid lines in FIG. 4, when a fault occurs within capacitor 10 resulting an excessive pressure inside capacitor 10, plug-like conductors 42 disengage from sockets 431 of connector 43 due to an expansion of bellows 41, thereby interrupting the electrical connection of 10 capacitor with the apparatus circuit.
The capacitor shown in FIG. 4 can be utilized in applications for a larger current duty. However, the capacitor is much more costly than any of the other designs mentioned above. Furthermore, all of the capacitors described above includes an internal interrupter located within the metal housing, which in itself is a significant manufacturing cost.